Keyed integrate and dump filter having crystal as integrator



1962 s. 1.. sTooPs ETAL 3,056,890

KEYED INTEGRATE AND DUMP FILTER HAVING CRYSTAL AS INTEGRATOR Filed June 25, 1959 Fig. 7 F 2 POSITIVE f FEEDBACK CRYSTAL K ISOQOUTPUT NEGATIVE FEEDBACK Fig. 4

CRYSTAL NEGATIVE 1 FEEDBACK F 9- 5 INVENTORS JOHN E. BENNETT SHERMAN L. STOOPS United States 3 056 890 KEYED INTEGRATEANb DUMP FILTER HAV- ING CRYSTAL AS INTEGRATOR Sherman L. Stoops, Hamburg, and John E. Bennett, Kenmore, N.Y., assiguors to Sylvania Electric Products Inc., a corporation of Delaware Filed June 23, 1959, Ser. No. 822,362 7 Claims. (Cl. 30788.5)

/ strictive rod, made of nickel or other suitable magnetostrictive material, the length of which is varied until it is resonant at the frequency of the tone to be detected. The rod is supported at its nodal point by a disc and the input signal is applied to a driving coil mounted about one end of the rod. An output coil is mounted about the rod at the other end, the entire assembly being mounted in a suitable container, usually cylindrical.

From this brief description of electro-mechanical filters, it will be readily apparent that they are relatively complex and expensive construction. Due to the basic nature of the construction, the resonator has a very limited frequency range, thus limiting the applicability of the filter. Moreover, since the resonant frequency of the resonator is dependent upon the dimensions of the magnetostrictive rod it is necessary, in order to maintain the frequency of the filter, to place the resonator in an oven to hold the temperature of the assembly at a predetermined temperature. A commercially available resonator of this type of which applicants are aware occupies a volume of 18 cubic inches, excluding the amplifier tubes and other circuitry associated with the resonator. To function as a keyed filter, in addition to the resonator three twin triodes and a pentode output amplifier are required. Thus it is seen that this system of the prior art in addition to being expensive, is bulky and heavy and requires considerable power for its operation. More important, however, is the systems inability to rapidly remove stored energy from the resonator, this characteristic precluding its use in communication systems employing high repetition rate. It has been observed that approximately one millisecond is required to remove undesired energy from the resonator, and since the dump period does not contribute to the detection of intelligence on the input signal, it is apparent that the filter limits the amount of intelligence that can be detected in a given period of time. For example, if a pulse repetition rate of 500 cycles is used, 2 milliseconds would be available for each bit of information; if one millisecond is required to dump the energy of the resonator between information bits, then only one millisecond is available for the build up of energy in the resonator, this being the period during which detection occurs.

It is an object of this invention to provide a filter inatent Q eluding a high Q energy storage element and means for rapidly removing the energy stored in the element.

Another object of this invention is to provide a keyed filter Which aifords a substantial reduction in space and weight over electro-mechanical filters.

Still another object of the invention is to provide a narrow band filter for a pulse type communication employing a high Q storage element wherein the energy stored in the element is rapidly removed at the end of each information bit by gated inverse feedback applied to the storage element.

Briefly, the filter in accordance with the invention employs a crystal, such as a Rochelle salt crystal, as the energy storage element for integrating the incoming signal. Energy is removed (dumped) from the crystal by means of a negative feedback path which is switched into the circuit at the termination of the information bit. With quenching initiated at the termination of the input pulse, the input signal during the quench period is insignificant with respect to the negative feedback signal, and

consequently it becomes unnecessry to switch out the input signal during this period. The circuitry associated with the crystal is entirely transistorized, contributing to a substantial saving in weight and volume, and additionally reducing the requirement for power to operate the circuit. The system is relatively insensitive to temperature, but even if an application should require maintenance of a particular temperature, the oven necessary for doing so is much smaller than that required for the electro-mechanical filters of the prior art.

Other features, objects and advantages of this invention will become apparent from the following description taken in conjunction with the acuompanying drawings, in which:

FIG. 1 illustrate-s a plurality of square wave intelligence carrying pulses;

FIG. 2 illustrates the response characteristic of a high Q crystal;

FIG. 3 illustrates the output of the filter system when the crystal is compensated to effect rapid removal of energy therefrom;

FIG. 4 is a block diagram of the filter circuit according to the present invention; and

FIG. 5 is a schematic circuit diagram of a preferred filter system in accordance with the invention.

With reference to FIG. 1, let it be assumed for explanatory purposes that a square wave signal carries in,

telligence in a communication system. For example, this might be a series of pulses, respectively of duration 1 t etc., which is formed from an RF carrier modulated so as to contain intelligence. To detect this intelligence requires that the incoming signal be integrated over the period of each successive pulse in a manner that the stored energy from one pulse does not deleteriously effect the detection of the next successive pulse. This may be accomplished by a narrow band resonator or filter of high Q, such as a crystal of Rochelle salt.

If energy in the form of a sine wave is applied to a narrow band filter, such as a crystal, the energy passes through the filter at an increasing exponential rate; i.e., the frequency response of a high Q resonator follows an exponential curve of the form: y=A(1e" where a is the time constant, t is time and A is the maximum amplitude of oscillation at the resonant frequency.

Patented Oct. 2, 1962 Should the sine Wave input signal suddenly be removed, there will continue to be an output from the crystal which decreases in accordance with the same exponential function. The length of time of rise and fall are equal, and both are a function of the quality of the crystal. If, however, radio frequency energy is applied to the crystal as a short pulse, for example of duration 1 which is short compared to the total exponential rise time, the rise envelope is almost linear in the initial portion, and exhibits a normal exponential decay after removal of the input pulse, as shown in FIG. 2. As was noted earlier, the intelligence to be detected is contained in the pulse itself, namely, during the period t in FIG. 2, with the decay period contributing nothing to the detection. It is therefore apparent that considerably more information could be detected in a given time if the output of the crystal were terminated at the tnstant of termination of the input pulse. In other words, if the energy stored in the crystal at the termination of the'period t couldbe removed instantaneously, the crystal would then be ready to detect the next pulse substantially immediately following the termination of the preceding pulse; Stated still another way, with rapid removal of the energy stored in the crystal, the repetition rate of the input pulses can be increased while maintaining the detection efiiciency of the crystal. The desired response is shown in FIG. 3, being triangular in shape instead of exponential, due to integration over only the initial portion of the exponential characteristic and rapid removal of the stored energy at the termination of the input pulse.

Referring now to FIG. 4, the characteristic shown in FIG. 3 is obtained, according to the present invention, by applying the input signal across a crystal 10, such as Rochelle salt, a high Q device, which integrates the signal for a predetermined period of time, approximately the duration of the input pulse. The output of the crystal is amplified in a highly degenerated amplifier 12 having constant gain and phase relationship, the output at terminal 14 being 180 out of phase relative to the input. At the appropriate time during the exponential rise portion of the response, the output appearing at terminal 14 is fed back through a gate circuit, shown diagrammatically as a switch 16, to the input to the crystal. This negative feedback signal rapidly reverses the build up and removes the stored energy from the crystal in a very short time. The system gain of amplifier 12 is substantia ly greater than the minimum desired dynamic range of the filter, and accordingly the input to the crystal need not be removed during the dump period of the crystal. When all of the energy is removed from the crystal 10, the gate or switch 16 is opened, opening the negative feedback path, to permit integration of the next succeeding pulse.

In some applications of the system it may be necessary is. linear only up to about one-tenth the 63% point, when low repetition rates are employed it may be necessary to alter the response of the crystal to maintain a linear characteristic throughout the integration period. In general, then, when the crystal buildup time i less than ten times the integration time, positive feedback is necessary. The positive feedback, when used, is applied over connection 20, but normally is so small that it likewise need not be gated off during the dump period.

Referring now to FIG. 5, a specific embodiment of the invention generally described in connection with FIG. 4 is shown. The input signal, for example the pulsed radio frequency signal illustrated in FIG. 1, is applied via input terminal 8 through coupling resistor 22 and capacitor 24 to the base of transistor T at a reference phase angle of 0. The amplified signal appearing at t e collector of the transistor is applied across crystal 10,

which may be a Rochelle salt crystal. The capacitance of the crystal and its holder, and such stray capacitance is there may be in the circuit, is compensated by a neutralizing capacitor 26 connected between the output terminal of the crystal and the emitter of transistor T The equivalent circuit of the crystal and its mount being essentially a series circuit comprising inductance, capacitance and resistance, the crystal integrates the amplified input signal in the manner described above, the integrated output being applied to the base of transistor T Transistors T T T and T and their associated circuitry constitute highly degenerated amplifier stages having substantially constant gain, and except for the stage including transistor T have substantially constant phase relationship. The final stage of amplification (T includes a tuned circuit in its collector circuit including the parallel combination of capacitor 28, resistor 30 and inductance 32 which contributes to the gain of the stage, and eliminates any tendency for high frequency oscillations. By appropriate selection of the elements of the tuned circuit a certain amount of phase correction may be introduced without appreciable sacrifice of gain of the stage. By reason of the five stages of application (or any other odd number) the output signal appearing at the collector of transistor T has a phase angle of relative to the input signal. The circuit thus far described, therefore, would prouce at output terminal 14 signals corresponding to that shown in FIG. 2 (exponential rise and exponential decay) for each input pulse of radio frequen cy energy.

As has been pointed out hereinabove, an important aspect of the present invention is the rapid removal of energy from the crystal 10 at the termination of each input pulse. To this end, at an appropriate time transistor T the base of which is connected through capacitor 34 to output terminal 14, is gated on by a suitable pulse applied to the base of the transistor. Filters of the keyed type being primarily useful in synchronous systems where the beginning and end of each information bit is known, a timing pulse generated at the end of each information bit may be employed to trigger a suitable gate pulse generator. For example, the gating pulse may be derived from a monostable circuit and applied to terminal 36. Since any gating circuit which opens to class A operation will have transients, it is necessary to insure that the spectral energy of these transients lies outside the pass band of the system. To this end, the normally rectangular pulse from a monostable circuit, which contains high frequency components, is modified or shaped to remove the high frequency components prior to its application to the transistor. As shown, the pulse is double integrated by resistor 38, capacitor 40 and resistor 42 and coupling capacitor 34' to provide a rounded pulse, the rate of change in amplitude of which is gradual throughout, for application to the base of transistor T The spectral energy of a pulse of this shape, termed a soft gate hereinafter in the specification and claims, is significantly below the lowest frequency crystal contemplated for use in the present filter application. Thus, the use of the soft gate allows the system an unlimited upper frequency as far as interfering transients from the gate are concerned.

When transistor T is rendered conducting by the gating pulse, the output appearing at the collector of transistor T (phase angle of 180) is applied to the base of transistor T The application of this negative feedback, at a time when the envelope wave form at the output terminal is approaching a maximum, rapidly removes the energy stored in the crystal and causes the output wave form to return to Zero very rapidly. In a circuit which has been satisfactorily operated, which was designed to have a minimum dynamic range of greater than 30 db, the system gain was sufliciently greater than 30 db that it was unnecessary to remove the input signal from the base of transistor T during the dump period of the crystal. In the event the initial Q of crystal is insuificient to give a perfectly linear build up, positive feedback may be applied from the input of the final stage of the amplifier over connection 20 through resistor 44 to the base of transistor T It has been found that the magnitude of the positive feedback signal required to insure proper integration characteristics for the crystal is of sutficiently small magnitude that it also need not be gated off during the dump period.

This circuit may be operated over a wide range of pulse repetition rates without circuit changes, for example between 20 cycles per second to 1000 cycles per second, with approximately the same fast dump time regardless of the repetition frequency. The dump time is about 200 microseconds thus allowing almost the total time of each pulse period for the integration function. The use of transistor circuitry contributes to low power consumption, the circuit is very definitely less expensive than the electro-mechanical resonators and associated circuitry, and the total volume occupied by the present system is approximately 10 cubic inches as opposed to the approximately 100 cubic inches for a system employing the electro-mechanical resonator. Additionally, this circuit, by merely changing the value of circuit components, can be designed to accommodate a very wide range of crystal frequencies (i.e., wide range of RP. frequencies in the pulses) in contradistinction to the electro-mechanical filter where each resonator is limited to use at a single frequency.

While there has been shown and described a specific embodiment of applicants invention, it will be apparent to those skilled in the art that many modifications and variations can be made within the scope and spirit of the invention as defined in the appended claims.

What is claimed is:

1. A wave shaping circuit including a resonator, said resonator comprising a crystal having input and output terminals, means for applying a pulsed radio frequency input signal to the input terminal of said crystal, an amplifier circuit including input and output terminals having its input terminal connected to the output terminal of said crystal, a feedback path connected between the output terminal of said amplifier and the input terminal of said crystal, and switching means in said feedback path operable in synchronism with said pulsed input signal for successively opening said feedback path for the duration of each input pulse and closing said feedback path for a predetermined short interval at the termination of each pulse for applying a quenching signal to the input terminal of said crystal to rapidly remove the energy stored therein during the duration of each pulse.

2. A wave shaping circuit comprising a high Q crystal having input and output terminals, means for continuously applying pulsed radio frequency input signals to the input terminal of said crystal, a degenerative amplifier including input and output terminals having its input terminal connected to the output terminals of said crystal, a feedback path connected between the output terminal of said amplifier and the input terminal of said crystal, and electronic switching means in said feedback path operable in synchronism with said pulsed input signal for successively opening said feedback path for the duration of each input pulse, and thereafter closing said feedback path for a predetermined interval at the termination of each input pulse for applying a 180 out-of-phase quenching signal to said crystal.

3. A wave shaping circuit comprising a high Q crystal having input and output terminals, means for continuously applying pulsed radio frequency input signals to the input terminal of said crystal, a degenerative amplifier including input and output terminals having its input terminal connected to the output terminals of said crystal, a negative feedback path connected between the out- 6, put terminal of said amplifier and the input terminal of said crystal, electronic switching means in said feedback path operable in synchronism with said pulsed input signal for successively opening said feedback path for the duration of each input pulse, and thereafter closing said feedback path for a predetermined interval at the termination of each input pulse for applying a out-of-phase quenching signal to said crystal, and a positive feedback path connected from said amplifier to the input terminal of said crystal for continuously applying a positive feedback signal to said crystal.

4. An electronic wave shaping circuit comprising a resonator, said resonator comprising a high Q crystal having input and output terminals, means for continuously applying pulsed radio frequency input signals to the input terminal of said crystal, an amplifier connected to the output terminal of said crystal and arranged to produce an output signal 180 out of phase relative to said input signals, a negative feedback path connected between the output of said amplifier and the input terminal of said crystal, and switching means in said negative feedback path including a normally non-conducting gating circuit operable in synchronism with said pulsed input signal to be rendered conducting for a predetermined interval at the termination of each input pulse for applying 180 out-of-phase quenching signals to said crystal.

5. A keyed filter system comprising a resonator, said resonator comprising a high Q crystal having input and output terminals, means for continuously applying pulsed radio frequency input signals to the input terminal of said crystal, a degenerative amplifier having an odd number of stages connected to the output terminal of said crystal and arranged to produce an amplified output signal 180 out-of-phase relative to the input signal, a negative feedback path connected between the output of said amplifier and the input terminal of said crystal, switching means in said negative feedback path including a normally nonconducting gating circuit and means for applying a rounded gating pulse to said gating circuit in synchronism with said pulsed input signal to cause said gating circuit to conduct for a predetermined interval at the termination of each input pulse whereby to apply 180 out-ofphase quenching signals to said crystal.

6. A keyed filter system for pulsed radio frequency signals comprising, a high Q resonator consisting of a crystal having input and output terminals, means for continuously applying radio frequency pulses to the input terminal of said crystal, a portion of the energy in each pulse being stored in said crystal and a portion being passed to its output terminal, a degenerative amplifier having input and output terminals, means connecting the output terminal of said crystal to the input terminal of said amplifier, said amplifier being operative to amplify the signal at the output of said crystal to produce a signal 180 out-of-phase relative thereto, a feedback path connected between the output terminal of said amplifier and the input terminal of said crystal, switching means in said feedback path including a gating circuit arranged to be opened during the period of each input pulse and to be closed for a predetermined interval synchronously with the termination of each input pulse to apply the output of said amplifier to the input terminal of said crystal for rapidly removing from the crystal the energy stored therein during each input pulse.

7. A keyed filter system comprising, a high Q resonator consisting of a crystal having an output terminal and an input circuit for receiving radio frequency pulses having a duration which is short relative to the total rise time of the exponential frequency response characteristic of said resonator, a portion of the radio frequency energy in each pulse being stored in said crystal and a portion being passed to its output terminal, an amplifier having input and output terminals, means connecting the output terminal of said crystal to the input terminal of said amplifier, said amplifier being operative to produce at its output 3,056,890 7 terminal; a signal 180 out-of-phase relative to the signal References Cited inthe file of this patent at the output terminal of said crystal, a feedback path UNITED, STATES PATENTS connected between the output terminal of said amplifier and the input circuit of said crystal, and a switching cir- 252x650 Spencer 13, I953 cuit in said feedback path arranged to be opened during 5 2,8813 17 Himke P 7, 1959 the period of each radio frequency pulse and to be closed 2,881,390 Wmn P 1959 for a. predetermined short time interval synchronously V with theterrninationof each radio frequency pulse to OTHER REFERENCES apply" the output of said'arnplifier to the input circuit of A Balanced Modulator Super-Regenerative Circuit, by

saidcrystal thereby to rapidly remove from thencrystal, 10 Roberts, QST, July 1932, pp. 19, 20. energy stored therein during each input pulse. 

